Method for the manufacture of pyrolytically coated filaments



. Jan. 28, 1969 H. s. SCHWARTZ 3,424,603

METHOD FOR THE MANUFACTURE OF PYROLYTICALLY COATED FILAMENTS Filed Oct. 6, 1965 ATTORNEY BY 9% QWXV INVENTOR. 43 HERBERT S. SCHWARTZ United States Patent 1 3,424,603 METHOD FOR THE MANUFACTURE OF PYROLYTICALLY COATED FILAMENTS Herbert S. Schwartz, Trotwood, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force Filed Oct. 6, 1965, Ser. No. 493,587 US. Cl. 117-4 Int. Cl. C23c 13/02, 17/02; B44c 1/22 7 Claims ABSTRACT OF THE DISCLOSURE The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to the formation of filaments composed in whole or in part of pyrolytic materials and particularly to a method for the simultaneous manufacture of a plurality of such filaments.

To provide structural reinforcement for a broad range of materials, especially resinous plastic compositions and in certain instances to achieve refractory and ablative capabilities in the highly oxidative, high temperature environments being increasingly encountered in aerospace vehicles and the rocket engines thereof, filaments with coatings deposited thereon by pyrolytic means are being increasingly sought after and used. In most of such applications it is preferred that the filament be of substantial length so that it may be continuously wound within the plastic component such as the walls of rocket engines and particularly of the combustion and nozzle segments thereof.

In order to achieve the desired substantial length, the prior art has developed a variety of relatively complicated apparatuses wherein a filamentous substrate such as a finely drawn tungsten wire is fed continuously through a deposition chamber where it is coated with a pyrolytic material which is in a vapor state within the chamber and, upon heating of the substrate, undergoes a gas phase plating reaction upon the substrate surface. In most preferred situations, the substrate is electrically heated by its resistance to the passage of electric current imparted at one or more points thereto while it is travelling within the deposition chamber. Because of the high temperatures encountered in many applications and in order to obtain precise control of the structure and dimensions of the coating being deposited, the deposition apparatus including the means for electrically energizing or otherwise heating the substrate are complex, expensive and of limited capacity.

It is accordingly an object of this invention to provide an improved method for the manufacture of pyrolytic filaments.

Yet another object of the invention is to provide a method for the simultaneous manufacture of a plurality of such filaments.

Still a further object of the invention is to provide a method for the manufacture of a purality of filaments from a single substrate which passes only once through a pyrolytic deposition chamber whereby the efliciency and output of such chamber are substantially increased.

Yet another object of the invention is to provide a 3,424,603 Patented Jan. 28, 1969 method for the simultaneous manufacture of a plurality of filaments composed entirely of refractory material.

Yet another object of the invention is to provide a method for the manufacture of pyrolytic filaments wherein the cross sectional dimensions of the finished products may be maintained within very close tolerances.

To achieve these and other objects and advantages which will appear from a reading of the following disclosure, the present invention teaches the deposition of the pyrolytic coating upon a strip-like substrate such as a metal ribbon or foil and thereafter dividing the strip along a plurality of longitudinal lines into a plurality of separate filamentary segments of substantial length corresponding to the length of the strip itself. To facilitate the ultimate division of the strip into the individual filamentary sections thereof, an important feature of the invention involves longitudinally scoring one or both sides of the strip-like substrate prior to the deposition of the pyrolytic coating thereon. It has been found that such grooves not only represent weaknesses in the substrate along which subsequent divisionmay be more easily achieved, but also, because of the angular disposition of the groove surfaces to each other and to the broadside surface of the substrate, provide growth sites from which crystallites of the material being deposited thereon will tend to develop in a haphazard rather than an orderly fashion. This random development leads to the formation of relatively large nodules of the pyrolytic substance which represent weaknesses in the pyrolytic coating along lines which are adjacent and coextensive with the substrate grooves as a result of which the ultimate division into filaments by separation at such grooves may be all the more readily achieved. Another factor believed to contribute to the ease of division is the tendency of the deposited coating to build up in layers of nucleation and cone formation in successive layers parallel to the substrate surface including the angularly disposed groove surfaces between which planes of cleavage are present. A modification of the invention involves the proper selection of the substrate material for a particular pyrolytic material so that the latter, upon its deposition, will dif fuse into the surface of the substrate resulting in a polycrystallinity constituting at least a nominal weakness in the composite material at the common surface between the substrate and the coating. This weakness facilitates removal of the coating from the substrate, either before, during or after it has been divided into the multiplicity of filamentary segments, as a result of which a plurality of filaments composed only of the pyrolytic material may be achieved.

To accomplish the division and separations toward which the above expedients contribute, the present invention also teaches the twisting or bending of the coated substrate in such a manner that the separation of the composite strip into individual filaments, composed either of a metal substrate coated on one or both sides with the pyrolytic material or entirely of the pyrolytic material, may'be accomplished without any cutting or machining and may in fact be accomplished so simply that the plurality of filaments held together as a single strip may be stored and transported as such for the sake of convenience and broken into individual filaments only at the time and place of their ultimate use.

The invention thus generally described may be more clearly understood from the following description of certain specific embodiments thereof in connection with which reference may be had to the drawings wherein:

FIGURE 1 is a fragmentary perspective view of a strip-like substrate prepared for use according to the method of this invention.

FIGURE 2 is an elevational view of a conventional pyrolytic deposition chamber of the type, the efficiency of which, the method of the present invention improves.

FIGURE 3 is an enlarged fragmentary cross sectional view of a pyrolytically coated substrate according to the present invention.

FIGURE 4 is a cross sectional view of a means for separating the strip into individual filaments.

FIGURE 5 is an enlarged cross sectional view of a composite metal substrate with a pyrolytic coating thereon formed according to the present invention.

FIGURE 6 is an enlarged cross sectional view of a filament composed entirely of pyrolytic material according to the present invention.

FIGURE 7 is an elevational view of a means for placing a coated strip according to the present invention under shear to separate the coating from the substrate.

FIGURE 8 is a cross sectional view of a composite strip-like substrate coated on both sides with a pyrolytic film prior to its division into individual filaments.

Referring now to FIGURE 1, the strip-like substrate 10 suitable for use in the method of this invention is shown to be a tungsten or other metal foil of generally rectangular cross section with rupper and lower broadside surfaces 11 and 12 resectively. At least one of these broadside surfaces and, in the case of the illustration in FIGURE 1, the upper surface 11, is provided with a plurality of laterally spaced, longitudinally extending, parallel V-shaped grooves 13. While these grooves may be cut or etched according to a variety of well known methods, particularly those employed in modern micro-circuitry, they are shown here to be provided by scoring with the diamond stylus 14 depending from the scribing arm 15 powered and guided by conventional means (not shown). Although the illustration of FIGURE 1 shows the subtrate 10 as being of substantial thickness, it is to be understood that, in view of the fact that most pyrolytic filaments are within a diameter range of from three to eight mils, the strip-like substrate must be extremely thin and is more truly in the nature of a ribbon or foil, the thickness of which varies from one to five mils. Consistent with these dimensions, the V-shaped grooves themselves are preferably of a depth of from one-half to three mils and are from one to three mils in width at the broadside surface such as 11 upon which they open. Depending upon the width of the ultimately desired filament, the grooves themselves are preferably spaced from three to five mils aparts transversely of the substrate.

The ribbon or foil thus scored may be stored on a supply roll such as 16 in the illustration of FIGURE 2 and from there led through a deposition chamber comprising the tubular housing 17, the vaporized pyrolytic material inlet tube 18 and the end openings 19 and 20. Within the deposition chamber, according to conventional preferred practice are positioned a plurality of longitudinally spaced electrodes 21 which, as a result of their association by the conductors 22 passing through opening 23 in the tubular housing 17 are energized by the power source such as the generator 24. The variable transformers 25 between successive .pairs of the electrodes 2.1 may be adjusted to vary the electric current passing through the segments of the substrate at any given instant lying between successive pairs of the electrodes. Accordingly, the current from one segment to the next may be increased to increase the heating of that portion of the substrate and the rate of pyrolytic deposition thereon. It is to be understood that the electrodes, though illustrated as comprising a point contact with the moving substrate, may be in the form of a variety of recently developed prior art units; and a significant factor is that, by only nominal modification, such electrodes may be adapted to maintain sliding electrical contact with the strip-like substrate of the present invention. Consequently, a plurality of filaments rather than a single strand may now be simultaneously coated by one pass through the deposition chamber thereby substantially increasing its capacity and output.

As best shown in FIGURE 3, the nature of the reaction between the vaporized pyrolytic material and the heated substrate is such that the pyrolytic material 26 such as graphite, a carbide, boride, nitride or the like plates out and builds up on the substrate 10. In the grooves 13 however, the build-up of the pyrolytic material, because of the angular disposition of the groove surfaces and of their intersection at a line at the apex of the groove, leads to the growth of crystallites along random paths which intersect to form relatively large nodules 27 within and above the grooved areas of the substrate. As a result of the weaknesses in the deposited coating resulting from the presence of these nodules which fortuitously coincide with the weaknesses in the substrate resulting from the notching thereof, and the angularity of cleavage planes in the coating the substrate strip emerging from the deposition chamber 17 in FIGURE 2 may be bent about an axis parallel to such grooves as shown in FIGURE 4 by being forced or pulled around a die member 28 having an arcuate surface 29, to break and divide it into a plurality of longitudinal sections. Such sections form the individual filaments such as 30 of FIGURE 5 comprising the metal substrate section 31 and the pyrolytic coating section 32 wherein the sides 33 of the filament are upwardly convergent in conformity with the side surfaces of the grooves originally formed within the substrate, In lieu of thus bending the strip about a longitudinal axis, individual filaments may be formed by twist ing the strip; i.e., by reversely rotating longitudinally spaced sections thereof about a longitudinal axis until the strip breaks into individual filamentary sections along the lines of the longitudinal scoring.

Where the relationship between the metal substrate and the pyrolytic material to be deposited thereon is such that the pyrolytic material tends to diffuse into the substrate surface to develop a polycrystalline structure at the juncture between the two which constitutes a relative weakness, sharp bending of the coated substrate, either in strip form or after individual filaments have been divided therefrom, will result in a separation of the pyrolytic material from the substrate so that a filament 34 as shown in FIGURE 6 composed entirely of the pyrolytic material will be obtained. In one specific example of this feature of the invention, the metal substrate may be composed of a metal with a high softening point such as tungsten and the deposition gas may be a methane series gas to deposit graphite upon and form a carbide with the tungsten or a boron halide such as boron trichloride with hydrogen gas to deposit a boron coating upon and form a boride with the substrate. One preferred method for so bending the composite substrate and pyrolytic film is illustrated in FIGURE 7 to represent the four-point, double-reverse bending mechanism comprising the rolls or guides 35, 36, 37 and 38 wherein the intermediate rolls 36 and 37 are displaced from the line of travel of the coated substrate 40 as it leaves the deposition chamber. The effect of this bending is to place the portion 39 of the coated substrate between the displaced guides 36 and 37 at or near the place of contact between the coating and the substrate so that the former is separated from the latter. Where any one or all of the guide rolls 35, 36, 37 and 38 are themselves arcuately formed transversely of the travel of the entire strip through the bending apparatus, the strip may be simultaneously bent about an axis parallel to the strip and the grooves thereon to divide it into individual filaments while at the same time placing them under shear to separate the coating from the substrate.

Where the filaments coated on both sides are ultimately desired, the substrate 41 as illustrated in FIGURE 8 may be provided with the notches 42 on its upper surface and 43 on its lower surface so that the weakening of the substrate in the plane of the apexes of the grooves will be enhanced, especially where the grooves of the upper surfaces are superimposed over the grooves of the bottom surface of the strip, and will result in the growth sites leading to the nodular formation in the same plane on both sides of the strip. Consequently, ultimate bending or twisting of the strip about an axis parallel to the lines of the grooves will result more easily and more definitely in the separation into individual filaments having the pyrolytic coating on both the top and bottom surfaces thereof.

While the present invention has been described in considerable detail in connection with certain specific embodiments and examples thereof, it is to be understood that the foregoing particularization is for the purpose of illustration only and does not limit the scope of the invention as it is defined in the subjoined claims.

I claim:

1. A method for the simultaneous manufacture of a plurality of pyrolytically coated filaments comprising the steps of:

(1) forming in a tungsten strip having upper and lower surfaces at least one groove in one of said surfaces;

(2) subjecting the grooved strip to a pyrolytic deposition atmosphere which consists of (a) a gas selected from the methane series or (b) a mixture of boron halide gas and hydrogen gas under such elevated temperature conditions whereby (a) a graphite or (b) a boride coating is formed on said tungsten strip; and

(3) separating the coated strip along said groove into pyrolytically coated filaments.

2. The method according to claim 1 wherein the pyrolytic deposition atmosphere is a mixture of boron trichloride gas and hydrogen gas.

3. The method according to claim 1 wherein the pyrolytic deposition atmosphere is a methane series gas.

4. The method according to claim 1 wherein the groove formed in the tungsten strip is formed by scoring.

5. The method according to claim 1 wherein the groove formed in the tungsten strip is formed by etching.

6. The method according to claim 1 wherein said groove is V-shaped and within the range of from one to three mils wide at its surface opening and from one-half to three mils deep.

7. The method according to claim 1 wherein there is more than one groove and wherein said grooves are laterally spaced by a distance of from two to five mils.

References Cited UNITED STATES PATENTS 744,245 11/ 1903 Semmer 225-2 1,937,858 12/1933 Taber 117-4 3,107,179 10/1963 Kohring 11746 X 3,130,073 4/1964 Van der Linden 117-46 X 3,206,322 9/1965 Morgan 117-4 RALPH S. KENDALL, Primary Examiner.

US. Cl. X.R. 

